Oxygen as electron acceptor

In view of the importance of the hydronium ion, HjO, and dissolved oxygen as electron acceptors in corrosion reactions, some values of the redox potentials E and chemical potentials n for the equilibria... 

The low specificity of electron-donating substrates is remarkable for laccases. These enzymes have high redox potential, making them able to oxidize a broad range of aromatic compounds (e.g. phenols, polyphenols, methoxy-substituted phenols, aromatic amines, benzenethiols) through the use of oxygen as electron acceptor. Other enzymatic reactions they catalyze include decarboxylations and demethylations [66]. 

Photocatalysis over oxide semiconductors has been proposed as an effective, mild and environmental friendly technique for the abatement of (refractory) organic contaminants with oxygen as electron acceptor [15]. In this case the reaction is normally exoergonic, as the overall reaction (mineralization) is organics + yC>2 = xCC>2 + ZH2O. 

An alternative mechanism for the oxidation of phenolic compounds is enzyme-catalyzed oxidation. Several classes of enzymes can catalyze this reaction. According to the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB), these enzymes are part of the E C. 1 class of oxidoreductases (see the Internet web site http //www.chem.qmul.ac.uk/iubmb/enzyme/ECl). The three main classes of enzymes that catalyze the oxidation of phenolic compounds are the oxidoreductases that use oxygen as electron acceptor (E.C. 1.10.3), the peroxidases (E.C. 1.11.1), and monophenol monooxygenase (E.C. 

Ruoff, P. Lillo, C. (1990). Molecular oxygen as electron acceptor in the NADH-nitrate reductase system. Biochemical and Biophysical Research Communications 172, 1000-5. 

Table 4.1 Reduction reactions using oxygen as electron acceptor catalyzed by heme proteins...

FIGURE 10.6 Relative energy yield during reduction of select electron acceptors with glucose as the electron donor. Values expressed as percent energy yield under aerobic (oxygen as electron acceptor) conditions. 

Bacteria and other microorganisms contribute to the oxidation of sulfur. Beggiatoa and Thio-thrix are species of bacteria that are capable of oxidizing hydrogen sulfide using oxygen as electron acceptor ... 

The metabohzation of methanol in yeasts begins with its oxidation to formaldehyde, catalyzed by an alcohol oxidase (AOX) that uses molecular oxygen as electron acceptor. From there, metabolism branches either to dissimilation via formate to CO2 with the production of two NADH as reduction equivalents, or to assimilation in the xylulose 5-phosphate cycle [31]. Oxidation and assimilation of methanol take place in specialized organelles, the peroxisomes, while dissimilation is a cytosolic process. Figure 19.1 illustrates all these steps... 

Oxidases use oxygen as electron acceptor and transfer two or four electrons. Those which transfer two electrons form H2O2. They include Glucose oxidase (see), the iron- and molybdenum-containing Xanthine oxidases (see), and the n-amino acid oxidases (containing FMN or FAD). The latter catalyse the irreversible formation of the corresponding a-ke-toacid. Oxidases which transfer four electrons contain copper they oxidize the substrate and form water, e.g. laccase, ascorbate oxidase and p-diphenol oxidase. 

NADH and NADPH oxidase (EC 1.6.99.1), a flavin en2yme using oxygen as electron acceptor for the oxidation of NADH with the production of hydrogen peroxide. 

The reaction carried out by n-amino acid oxidase uses molecular oxygen as electron acceptor efficiently. The conventional assay of this... 

Succinoxidase. Washed particles from heart muscle oxidize succinate to fumarate with oxygen as electron acceptor, This is the so-called succinoxidase preparation. The initial step in the oxidation of succinate has recently been found to be catalyzed by an iron-flavoprotein. When this is isolated from other components of succinoxidase, it fails to react with other enzymes and reduces efficiently only phenazines of the several oxidants that are effective with more complex preparations. Partially fragmented particles have been obtained by differential centrifugation of isobutanol-treated preparations of larger particles from mitochondria or by digestion with trypsin in the presence of cholate. These treatments yield preparations that can reduce cytochrome c. ... 

Although purified animal enzymes appear to have broad substrate specificity, extremely specific oxidases have been found in bacteria. Achromonas sp. was found to produce oxidases when incubated with histamine or putrescine. The enzymes induced by one amine were incapable of reacting with the other. These enzymes are able to use indo-phenols as well as oxygen as electron acceptors, but they are inhibited by methylene blue. 

Among the mammals, the relative efficiencies of NAD" and oxygen as electron acceptors for xanthine oxidase varied not only from species to species but also from tissue to tissue, A possible explanation of this tissue variation was suggested by the observation that aging rat liver homogenates resulted in an increase in the activity with oxygen and a decrease in the activity with NAD ... 

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